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Duet Song and Sex Roles During Territory Defence in a Tropical Bird, the Black-Bellied Wren, Thryothorus Fasciatoventris

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Duet Song and Sex Roles During Territory Defence in a Tropical Bird, the Black-Bellied Wren, Thryothorus Fasciatoventris ANIMAL BEHAVIOUR, 2004, 68, 721–731 doi:10.1016/j.anbehav.2003.10.026 Duet song and sex roles during territory defence in a tropical bird, the black-bellied wren, Thryothorus fasciatoventris DAVID M. LOGUE & DAVID E. GAMMON Department of Biology, Colorado State University and Smithsonian Tropical Research Institute, Republic of Panama (Received 18 February 2003; initial acceptance 11 August 2003; final acceptance 21 October 2003; published online 23 August 2004; MS. number: A9557) A diverse array of bird species show a behaviour known as duet singing, in which mated pairs sing temporally coordinated songs. Several studies have shown that simulating territory intrusion with conspecific song playback evokes duet song from duetting bird species, but the adaptive significance of coordinated song during territorial defence remains unclear. The function of duetting is further obscured by our poor understanding of the roles of the sexes in territory defence among year-round territorial species. We developed a simple optimality model that predicts the conditions under which intersexual territorial defence is most likely to occur. We used male solo song and female solo song playback as well as a novel method called ‘stereo duet playback’ to test the predictions of the model and to explore the function of song initiation and song answering during territorial encounters. We conducted these experiments in the field on the black-bellied wren, a year-round territorial passerine. Birds of both sexes responded to all treatment types. Males initiated more songs during opposite-sex playback than during same-sex playback and both sexes were more likely to answer their mates’ songs when mates were physically closer. We argue that the acoustic mate-guarding hypothesis does not adequately account for these results, and suggest that duetting during territorial encounters allows mates to identify one another, thus preventing intrapair aggression. As predicted by our model, females showed a strong same-sex bias in territory defence, whereas males approached playback of both sexes. Also in support of the model, males with dependent juveniles showed stronger intersexual territoriality than males not involved in a breeding effort. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Avian vocal duetting is especially common in species in versus heterospecific playback (Levin 1996b). Magpie lark, which a mated male and female defend a shared territory Grallina cyanoleuca, pairs coordinate a higher proportion of throughout the year (Farabaugh 1982). Although the their songs to form duets during conspecific playback adaptive significance of this behaviour varies among versus prior to playback (Hall 2000). Similar results have species, sexes (Levin 1996b) and contexts (Sonnenschein been found for stripe-backed wrens, Campylorhynchus & Reyer 1983), simulated territory intrusion evokes height- nuchalis (Wiley & Wiley 1977), usambiro barbets, Trachy- ened levels of duet song in many species of year-round phonus usambiro, slate-coloured boubous, Laniarus funebris, territorial birds. Pairs of robin chats, Cossypha heuglini, duet and striped kingfishers, Halcyon chelicuti (Wickler 1976). In when presented with conspecific mounts and playback all of these experiments, an elevated level of duet song (Hultsch & Todt 1984). Female bay wrens, Thryothorus during playback was associated with other measures of nigricapillus, initiate song more often and are more likely to agonistic response (e.g. flights, approach to speaker). form duets by answering their mates during conspecific It is clear that pairs duet more during simulated territorial intrusions, but what is the adaptive significance of coordinated song to individual birds? One obstacle to Correspondence: D. M. Logue, Department of Biology, Colorado State answering this question is our poor understanding of University, Fort Collins, CO 80523-1878, U.S.A. (email: dlogue@ territory defence in year-round territorial birds (see Table 5 lamar.colostate.edu). in Hall 2000 for a review of duetting and joint territorial 721 0003–3472/03/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. 722 ANIMAL BEHAVIOUR, 68, 4 defence). Several studies have reported a strong same-sex intersexual territoriality when retention of the current bias in territorial defence (Greenberg & Gradwohl 1983; mate stands to provide high fitness benefits. This is likely Freed 1987; Levin 1996b; Morton & Derrickson 1996; Hall to be the case during the period of nestling and juvenile 2000; Bard et al. 2002; Seddon et al. 2002). The evidence care if stepparents do not aid in raising the young (as in presented in such studies typically shows that birds female black-bellied wrens; D. M. Logue, unpublished respond more strongly to intrasexual versus intersexual data) or if they commit infanticide (as in male house solo song playback. All of these studies, however, also wrens, Troglodytes aedon; Freed 1987). revealed that mated birds respond physically (e.g. by The model does not address mate following during approaching the speaker) to opposite-sex playbacks. It opposite-sex playback as a means of mate retention, has been suggested that this apparent intersexual territory although in some cases it may provide a simpler explana- defence represents intersexual protection against infanti- tion for the observed pattern of intersexual approach to cide (Freed 1987) or a means of mate retention when the playback speaker. In the interest of keeping the model unattended individuals are likely to advertise for a new simple, we did not consider the cost represented by an mate (Morton & Derrickson 1996). injury to the mate who has defended alone. Including We developed a simple model that predicts the con- such a cost would increase the conditions under which ditions under which intersexual territory defence should intersexual defence is less costly than the alternative. The occur. The model assumes strict social monogamy; either model applies equally well to a pair of intruders, rather an intruder is rebuffed or it replaces the same-sex territory than a single intruder, if we bear in mind that the cost C to holder. Consider the case in which a solo female intrudes the focal individual is likely to be greater if the individual upon a mated pair’s territory. The paired male will have also must defend against a same-sex intruder. Until now, it a lifetime fitness of V1 if he keeps his current mate and has been impossible to address the question of intersexual a fitness of V2 if the intruder becomes his mate. If the male territoriality during simulated pair intrusions because does not attempt to drive the intruding female from his traditional duet playbacks broadcast both male and female territory, there is a probability p that she will achieve duet contributions from a single speaker. We developed residence on the territory and become his mate. We a method of broadcasting duet recordings from two assume that if he does help to defend the territory against sources to more realistically simulate a duetting pair. the intruder, he will retain his current mate. Finally, We conducted song playback experiments on free-living defending the territory against the intruding female black-bellied wrens to address the following questions. (1) imposes a cost C to the male. With these assumptions, How and why do these birds initiate song and answer we can generate a simple model that predicts that a mated their partners’ songs to form duets during territorial male will optimize his fitness by defending his territory intrusions? (2) Does either sex show intersexual territorial against intruding females when defence? Black-bellied wrens maintain all-purpose territo- ries throughout the year. Both males and females sing repertoires of sex-specific song types, some of which they V1 ÿ C > pV2Cð1 ÿ pÞV1 share with other individuals in the population. These The left side of the inequality represents the fitness payoff songs may be sung independently or coordinated with the to the male if he defends the territory against the intrud- song of the mate to form a duet. In contrast to the ing female. The right side of the inequality represents his congeneric bay wren (Levin 1996b), both sexes of black- average fitness payoff if he does not defend; the average bellied wrens initiate duets. Individuals typically alternate duet contributions to form duet trains (sensu Brown & payoff if the male does not defend will depend on both V1 Lemon 1979) that vary in length from two to 16 and V2 because either female could potentially occupy the territory after the conflict. The inequality can be simpli- contributions. fied to yield Male black-bellied wrens are substantially larger than females (all Ns > 21; Xmass: 28.0 g versus 22.3 g; Xtarsus: 26.7 mm versus 24.0 mm; Xwing: 67.7 mm versus V1 ÿ V2 > C=p 63.5 mm, respectively). Unmated males regularly obtain and hold solo territories. We have no evidence that This model suggests that a male should defend his females naturally acquire or maintain solo territories, territory against an intruding female given the conditions nor do we have evidence that older offspring help in the that (1) the male assesses that his fitness with the current rearing of younger offspring. Social monogamy is the only mate would exceed his fitness with the intruder and (2) known mating system in this species. Both sexes are the cost of intersexual territory defence is sufficiently low. highly active in nest building and nestling and juvenile Of course, one could apply the model to a mated female care (personal observation). during a solo male intrusion to make predictions about intersexual territorial defence by females. For species in which one sex is larger than the other, we METHODS expect that the larger sex will pay lower costs for in- tersexual territoriality than the smaller sex. Therefore, all Study Population else being equal, the model predicts that in species where one sex is larger, the larger sex is more likely to practice We performed a series of field playback experiments on intersexual territoriality.
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